1. Field of the Invention
The present invention relates to an image reading apparatus, in particular, an image reading apparatus for reading from above a document laid facing upward.
2. Description of the Related Art
When an image exists on the back side of the reading surface, it occurs sometimes that the image on the back surface can be seen through the paper. Thus, it is difficult to determine the base brightness of the reading surface accurately and impossible to express the detected image in a proper density. This see-through phenomenon of the back surface image will be called hereinafter as "show-through". The show-through phenomenon is more conspicuous when the thickness of the paper is thin. When there is an error in detecting the base brightness, the density control becomes inaccurate and may cause whitening of characters thus making them illegible.
The common practice used in copying machines to avoid this show-through is to eliminate any images below a certain density level, thus making it possible to maintain the base brightness constant and to remove show-through images. However, if the density of the image on the reading surface is too low, the elimination of all images below a certain level uniformly can cause a problem of inadvertently eliminating a necessary portion of the images on the reading surface.
For example, Japanese Patent Unexamined Publication JP-A-07-87295 (1995) discloses a technique to solve such a problem. Particularly, it determines an accurate base density by multiplying the back surface image data with a certain ratio to obtain the back surface component, and subtracting the component from the reading surface image data to extract only the actual content of the reading surface.
Further, Japanese Patent Unexamined Publication JPA-05-183749 (1993) discloses another technique to solve such a problem based on the assumption that the density distribution of the background pixels would be a normal distribution. In detail, it determines the density range of the background using the mode "mm" of the density histogram as a reference, and separates the image part from the background using "2.times.m" as the threshold value or the upper limit of the density range.
In yet another technology disclosed by Japanese Patent Unexamined Publication JP-A-08-237485 (1996), a brightness histogram was prepared and the base area was defined as the vicinity of the mode of the histogram. In other words, a brightness slightly lower than the brightness that constitutes the mode is used as the boundary for separating the image part from the background.
However, the above-mentioned technologies have the following problems.
First, the technology disclosed by Japanese Patent Unexamined Publication JP-A-07-87295 (1995) requires an accurate alignment of the image positions on the reading surface and on its back surface before two images are compared. In case of automatically transferring a paper into a position on a platen glass, turning it over using an auto document feeder and reading the back surface of the paper, the positions of the front surface and the back surface are approximately equal. Because the paper consists of single sheet and is thin. Therefore, it is possible to align the image of the front surface with the image on the back surface and compare the two images accurately.
On the other hand, in case of reading spread pages of a document consisting of multiple pages, such as a book or a file with some thickness, from above, the reading surface is composed of the right and left pages, and the right page of the reading surface is the back surface of the left page of the next reading surface, and the left page of the reading surface is the back surface of the right page of the previous reading surface. If the number of pages on the right and left sides is different, the height of the surface is different between the right and left sides, and the image of the front surface and the image of the back surface have different positions. Consequently, it is difficult to accurately align the positional relation between the images of the reading surface and the back surface. In conclusion, this technology presents a problem for a document consisting of multiple pages, such as a book or a file with some thickness, that it cannot extract the image of the reading surface accurately.
The technologies disclosed by Japanese Patent Unexamined Publications JP-A-05-183749 (1993) and JP-A-08-237485 (1996) assume that the area of the image data where the mode of the density or the brightness exists is the base area. As a result, if there is a strong show-through, the mode can be constituted of the density or brightness of the show-through area. In other words, since the mode does not represent the density or brightness of the base, it can cause an error in detecting the density or brightness of the base.